Earth-boring tools for forming boreholes in subterranean earth formations, such as for oil and gas extraction, carbon dioxide sequestration, etc., often include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include cutting elements fixed to a bit body of the drill bit. Earth-boring tools include, but are not limited to, core bits, bi-center bits, eccentric bits, hybrid bits (e.g., rolling components in combination with fixed cutting elements), roller cone bits, reamer wings, expandable reamers, and casing milling tools. As used herein, the terms “earth-boring tool” and “drilling tool” encompass all of the foregoing, and equivalent structures.
Cutting elements for earth-boring tools may include a body of polycrystalline diamond. Such cutting elements are often referred to in the art as “polycrystalline diamond compact” (PDC) cutting elements, and often include a volume of polycrystalline diamond that is formed on an end of a supporting substrate. PDC cutting elements formed on a substrate commonly comprise a thin, substantially circular disc of polycrystalline diamond (although other configurations may also be used), commonly termed a diamond “table,” which includes a layer of polycrystalline diamond. Polycrystalline diamond includes diamond grains (i.e., crystals) that are bonded together by direct inter-granular diamond-to-diamond bonds. The direct inter-granular diamond-to-diamond bonds are formed by subjecting the individual diamond grains to what is referred to in the art as a high-temperature and high-pressure (HTHP) process, while the diamond grains are in the presence of a metal solvent catalyst (e.g., a Group VIII metal such as iron, cobalt, or nickel). Upon forming the polycrystalline diamond, the metal solvent catalyst may remain in interstitial spaces between the interbonded diamond grains. At least a portion of the polycrystalline diamond is employed as a cutting edge to cut the subterranean formation being drilled by a drill bit on which the PDC cutting element is mounted.
The presence of the metal solvent catalyst in the interstitial spaces within the polycrystalline diamond may lead to thermal degradation of the polycrystalline diamond commencing at about 400° C. due to differences in the coefficients of thermal expansion (CTEs) of the diamond and the catalyst. Beginning at temperatures of around 700° C. to 750° C., the catalyst may convert diamond to graphitic forms of carbon. Such temperatures may be reached within the polycrystalline diamond in a PDC cutting element during drilling of a formation due to the friction between the PDC cutting element and the formation.
To avoid such thermal degradation, it is known to remove catalyst material from the interstitial spaces between the interbonded diamond grains in polycrystalline diamond material using acid-leaching processes. In such processes, at least a portion of a body of polycrystalline diamond may be immersed in an acidic solution containing hydrofluoric acid, hydrochloric acid, nitric acid, mixtures of acids, etc. Examples of such processes are described in, for example, International Publication Number WO 2007/042920 A1, published Apr. 19, 2007, and entitled “Method of Making a Modified Abrasive Compact,” the entire disclosure of which is hereby incorporated by reference.
Removal of the catalyst from the polycrystalline diamond, particularly at the surfaces thereof that will contact the formation during use, reduces the tendency of those portions of the polycrystalline diamond to degrade during drilling. However, removal of substantially all of the catalyst from the polycrystalline diamond may render the polycrystalline diamond less tough and less resistant to fracture, which may be particularly undesirable in certain drilling applications.